Graphite structures in nuclear reactors



April 16, 1968 H. BRIDGE ETAL. 3,378,459

GRAPHITE STRUCTURES IN NUCLEAR REACTORS Original Filed Jan. 19, 1963 3Sheets-Sheet 1 mumm rmpzmmm "a 0 per P 1968 H. BRIDGE ETAL 3,378,459

GRAPHITE STRUCTURES IN NUCLEAR REACTORS Original Filed Jan. 10, 1963 3Sheets-Sheet r {ma/res) AprillG, 1968 H. amnes- ETAL GRAPHITE STRUCTURESIN NUCLEAR REACTORS Original Filed Jan. 10, 1963 3 Sheets-Sheet s Q v Msaw \Q United States Patent 3,378,459 GRAPHITE STRUCTURES 1N NUCLEARREACTORS Harry Bridge and Brian Thomas Kelly, Culcheth, near Warrington,England, assignors to United Kingdom Atomic Energy Authority, London,England Continuation of application Ser. No. 250,599, Jan. 10, 1963.This application June 23, 1966, Ser. No. 560,017 Claims priority,application Great Britain, Jan. 12, 1962, 1,212/62; Feb. 7, 1962,4,806/62 7 Claims. (Cl. 176-84) ABSTRACT OF THE DISCLOSURE In a graphitemoderator structure which is to be exposed to irradiation by neutrons ina nuclear reactor during operation and to be subjected to determinedtemperature distribution during normal operation, there is placed ineach region of the structure a graphite whose coeflicient of thermalexpansion in at least one given direction is selected in accordance withthe normal operating temperature in that region to dimensionallystabilize the graphite in such direction under neutron irradiation.

The present application is a continuation of our copending applicationSer. No. 250,599, filed J an. 10, 1963, and now abandoned.

The present invention relates to graphite structures for nuclearreactors. Such a structure is employed for the moderator in thermaltypes of nuclear reactor.

When graphite is subject to irradiation by neutrons, the collisionswhich occur between the neutrons and the carbon nuclei give rise to thedisplacement of carbon atoms from their lattice positions.Microstructurally, graphite is constituted by parallel planarcrystallities and the lattice defects resulting from displacements byneutrons are therefore divisible into interstitial accumulations betweenthe crystalliate planes and vacancies in the planes themselves. Theinterstitial accumulations promote a tendency to increase the spacingofthe planes so that in the direction perpendicular to the planes, knownas the c axis direction, a dimensional increase occurs. In the directionof the planes themselves, known as the a axis direction, the Vacanciesand puckering of the layer planes lead to a dimensional contraction.

The dimensional changes in the microstructure have a similarmanifestation macroscopically. In orientated material, layer planesoccur in the case of extruded material parallel to the direction ofextrusion and in the case of pressed material perpendicular to thedirection of pressing. Thus, the bulk dimensional change induced byneutron irradiation in orientated material, often referred to as theWigner growth, has been generally one of expansion perpendicular to thegrain and contraction in the same direction as the grain.

Hitherto, it has been necessary that the graphite which is built up toform a nuclear reactor moderator structure is allowed freedom to expandor contract under irradiation. Elaborate features have had to be evolvedin order to accommodate the amounts of irradiation-induced expansion andcontraction which will occur over the working life of the reactor.Furthermore, in coherent forms of graphite, different neutron fluxes atdiiferent points give rise to unequal rates of dimensional change andhence the possibility of the material becoming deformed and'cracked.Great advantages are therefore to be gained if the irradiation-induceddimensional changes of the graphite can be significantly reduced, orbetter still eliminated entirely.

It has already been recognised that the net effect on graphite ofneutron irradiation results from a balance between the rate ofproduction of damage by the neutrons and a concurrently acting processof self-recovery which is due to annealing by thermal activation. Forthis reason, the irradiation-induced dimensional changes are dependentupon temperature.

According to the present invention, in the building of a graphitestructure which is to be exposed to irradiation by neutrons in a nuclearreactor during operation of the latter and to be subjectto apredetermined temperature distribution when such operation is normal,there is placed in each region of the structure a graphite whosecoeificient of thermal expansion in at least one given direction isselected in accordance with the normal operating temperature in thatregion with the objective of dimensionally stabilising the graphite inthat direction under neutron irradiation.

The invention is based on the finding that an expression relatingthermal expansion properties and irradiation-induced dimensional changesis valid if use is made of bulk measurements of the latter as distinctfrom measurements of the crystal structure for example, by an X-raydiffraction technique. Therefore, it has now become possible, using inthis expression direct measurements taken from graphite samplesirradiated at constant temperature, to determine a coeflicient ofthermal expansion which will give a high degree of dimensional stabilityunder irradiation at that temperature.

In all probability there will be diiferences between the neutron energyspectrum prevailing during the irradiation of the samples and thatdesigned for in the graphite structure in question. Such difference ofspectrum in distribution and intensity implies some difierence in therate of production of damage in graphite. An appreciable difference ofthe respective neutron spectra can therefore preclude direct applicationof the thermal expansion coefficients for dimensional stability on thebasis of true tem- .peratures if the maximum degree of dimensionalstability is being sought.

To meet these circumstances, the invention provides that in the buildingof a graphite structure as previously specified there is placed in eachregion of the structure a graphite whose coefiicient of thermalexpansion in at least one given direction is selected for dimensionalstability in that direction under the neutron irradiation by (a)correlating the temperature of normal operation of that region with theirradiation temperature required to produce the same net damaging effecton graphite under the irradiation conditions of chosen availablegraphite irradiation data, (b) estimating for such irradiationtemperature the bulk dimensional changes indicated by said data, and (c)using these dimensional changes to determine the requisite coefiicientof thermal expansion.

This method presupposes that a temperature correction can be asufiicient compensation for ditferences of neutron spectrum. As a resultof the annealing process mentioned previously, it is deduced subject tocertain assumptions that the same net damaging etfect is produced ingraphite with two different flux spectra if the respective irradiationtemperatures are related by an expression derived from annealingkinetics. It is therefore on the basis of such an expression that thetemperatures are correlated for the purpose of the method above setforth.

The determination of the requisite coefficient of thermal expansion ismade with the following equations by which the macroscopic thermalexpansion coefiicient a in a given direction x is related to the rate ofdimensional change X in the same direction:

where ar is the thermal expansion coeflicient of a graphite crystal inthe c axis direction,

01 is the thermal expansion coeflicient of a graphite crystal in the aaxis direction,

x is the rate of dimensional change of a graphite crystal in the c axisdirection,

x is the rate of dimensional change of a graphite crystal in the a axisdirection, and

A is a constant in respect of the direction x for a particular qualityof polycrystalline graphite.

a (for stability) a,(for stability) X per C.

The values of the ratio 5 to be used in this equation are obtained bycalculation from the measurements of bulk dimensional changes andthermal expansion coefficients indicated by the chosen irradiation data,Equations 1 and 2 being used for this purpose. It appears fromirradiations to date that the ratio 6 does not change appreciably withirradiation dose and therefore is conveniently regarded as a constantfor any particular temperature.

In order that dimensional stability under neutron irradiation mayprevail in all directions, it is preferred that the graphite selectedfor the structure has a random crystal orientation of highcrystallographic perfection and is substantially isotropic at least tothe extent of the thermal expansion properties. It is therefore afeature of the invention that the structure is built with substantiallyisotropic graphite, that is to say, graphite with an isotropy ratio inrespect of the thermal expansion properties which is less than about1.3.

So that the application of the invention may be explained in greaterdetail for one particular example, the relevant graphite irradiationdata chosen for this purpose will be set out first of all:

The origin of the chosen data is a series of irradiations at differentconstant temperatures in a high flux materials testing reactor of theheavy water moderated and cooled type of samples of so-called grade Areactor quality graphite. The neutron activation of nickel by thereaction 58Ni (n, p) 58Co provides, in the absence of any wellestablished method of measuring neutron energy spectra, a good monitorfor fiux in the energy range damaging to graphite and conversion ofresulting activity into the nickel flux (p is made using a cross sectionof 0.107 barn. The high flux irradiations, however, favour the use ofthe similar reaction 59Co (n, 'y) 60Co which is convertible using across section of 33.0 barns to 95 and then with an experimentallydetermined factor to the equivalent nickle fiux. The following tablegives results calculated from the measurements of bulk dimensionalchanges and thermal expansion coefficients using Equations 1 and 2:

Calculated percentage rate of change of crystal dimensions per 10 nickelflux 6 In n Tempierature, degrees Centi- The tabulated values of 6enable a determination by Equation 3 of the coeflicient of thermalexpansion required in the graphite for dimensional stability at eachtemperature in the flux spectrum of the particular testing reactoremployed. However, it is preferred for the purposes of the presentexample that this information is available for the dilferent fluxconditions of a graphite moderated reactor since correlation in thedesigning of a graphite moderator structure is thereby simplified. Theinformation is accordingly related to a standard unit of neutron fluxdefined by the flux produced at the wall of a fuel element channel in aselected graphite moderated reactor when adjacent fuel of specified formis generating heat at an arbitrarily fixed figure expressed in terms ofmegawatts per adjacent tonne (mw./Ate.). This standard unit completelydefines a rate of irradiation damage for graphite at the channel wallwhich is taken as the reference point. One then finds the temperaturewhich would have to prevail notionally at this reference point toproduce the same net damage effect as has been produced in the testingreactor for equal irradiation doses, this temperature being referred toas the equivalent temperature.

The previously tabulated testing reactor results have been related onthis basis to a reference point in a Calder- Hall reactor at 3.12mw./Ate. as follows:

In the accompanying drawings, FIGURE 1 shows a curve drawn through aplot of a against the equivalent temperature.

The particular example now to be considered is a nuclear reactorgraphite moderator structure which overall approximates to the shape ofan upright right cylinder and is penetrated from top to bottom bycircular constantsection channels arranged vertically in parallelrelationship on a square pitch. For present purposes, it is assumed thatnuclear fuel disposed centrally in the channels extends continuouslywithout variation of cross section from top to bottom of the moderatorstructure. Through the annular space between the fuel and the channelwalls of the flow of a gaseous coolant, taken in the present example tobe carbon dioxide, is in the upward direction. Consequently there is avertical temperature gradient increasing with height; it is thevariation of temperature due to this gradient of which account is takenin the present example.

A temperature distribution for normal operation based on 300 C. at themid-horizontal plane is shown in the following table for elevenequispaced levels unmbered -5 to 5 and two intermediate levels 4.5 and4.5 midway between the 4 and -5 levels and the 4 and 5 levelsrespectively.

The above tabulated temperatures are corrected to equivalenttemperatures by means of the following equation derived from annealingkinetics:

(i l E T. T E gt 3.12 (4 T is the true absolute temperature T is theequivalent absolute temperature,

K is Boltzmanns constant (8.61 x 10- en./ K.), E is the activationenergy for annealing, and

P is the equivalent power rating.

where The activation energy is assumed for present purposes to have asingle value on the basis that annealing takes place in a sequence ofprocesses controlled by initial behaviour. The value adopted istherefore applicable to the initial stages of annealing and isdetermined by experiment. Experimental results have varied, but thevariation is within limits consistent with the degree of accuracy to beexpected of this procedure of moderator construction. The value used inthe present example is 1.2 ev.

It will be noted that Equation 4 calls for the power rating in themoderator structure under consideration to be expressed in terms of anequivalent rating; since reactor geometries and fuels vary, the powerrating in one reactor will not necessarily be in the sameproportion tothe graphite damage rate as in another reactor. The conversion toequivalent rating serves to put the rating of the one reactor in thesame proportion to damage rate as in the other or reference reactor. Useis made of a relationship whereby the damage rate in an infinitegraphite mass at a distance r from a uniform infinite line source offission neutrons is proportional to the function Account is taken of theeffect of voids in the graphite structure by rewriting this function aswhere r is the distance through graphite. The rate of damage to thegraphite in the moderator can be obtained by summing the damage due toeach fuel channel, each such channel being regarded as an infinite linesource of fission neutrons. Thus, the damage rate at a point is =CW 1; dTi 9 a) where r, is the distance from O of the i fuel channel r is thedistance from the fuel channel to 0 through graphite and W is the powerrating per unit length of fuel.

Now, the power rating per unit length of fuel may be consideredproportional to APp Where A is the crosssectional area of the fuel, Pthe power rating in mw./Ate. of uranium, and M the density of uranium inthe fuel in Therefore, for finding the equivalent power rating P withrespect to the reference point in the Calder-Hall reactor,

where P is the local power rating of the moderator structure underconsideration.

The effect of different graphis densities may be allowed for y replacingMug) y 1'g) Where 1'g=(p0)ug p being the actual density of the graphiteand po a standard density. The function (r has been found experimentallyfor a standard density of 1.6 gms./cc. and is shown in the accompanyingFIG. 2. For the reference point in the Calder-Hall reactor, the fuelcross sectional area is 6.66 sq. em. and the summation is found to be1.32 for a graphite density of 1.75 gms./cc. Consequently, if A, thecross sectional area of the fuel to be used in the moderator structure,is expressed in sq. cm., the complete expression for this structure is:

o Pu l s.79 18.7 r. (6)

the figure of 18.7 being the uranium density applicable to the referencepoint.

The local power rating for normal operation is found at each level ofthe moderator structure under consideration and is converted to theequivalent rating using Equation 6 and the function of FIG 2. With theequivalent rating figures, the equivalent temperatures are found bymeans of Equation 4 and the coefi'icients of thermal expansion fordimensional stability read off the curve of FIG. 1. The following tablegives the results for the moderator structure under consideration on theassumption that in this case Equation 6 reduces to P =0.954P

Equivalent E uivalent a 10 Moderator Local lfower Power lemp X or 0Level Rating Rating degrees stability mwJAte. Centigrade 5 0 0 4. 5 0.97 0. 93 442 4. 35 4 l. 98 l. 86 414 4. 90 3 3. 68 3. 52 378 5. 45 25.19 4.96 343 5.40 l 5.88 5. 60 316 5.15 0 6. 30 6. 0 285 4. 50 -1 5.885.60 257 3.60 2 5. l9 4. 96 236 2. 90 -3 3. 68 3. 52 217 2. 50 4 1. 981.86 213 2. 50 4. 5 0. 97 0.93 221 2. 60 --5 0 0 00 Following the abovetabulated results in the building of the moderator structure, from thebottom to a height between the intermediate level 4.5 and the level 4the graphite blocks have a thermal expansion coefiicient of 2.60 10- C.,in the next layers up to between the levels 3 and -2 the blocks have athermal expansion coefficient of 2.50 10- C., and so on. The quality ofthe graphite employed is to be polycrystalline of high crystallographicperfection and substantially isotropic, the specified thermal expansioncoefiicient being the average over the temperature range l96 C. to 20 C.in this case.

As an alternative to the treatment in layers of uniform thickness, thelayers may have varying thickness, especially to meet the case wherethere is considerable variation of the equivalent temperature gradientand graphites are available with a wide variety of thermal expansioncoefficients covering the required range in small steps. In general, themoderator structure is divided into regions each defined by isothermscorresponding to the limits of preselected temperature ranges coveringthe full compass of the temperature distribution, and a particularthermal expansion coeflicient to minimise irradiation-induceddimensional change is specified for isotropic graphite to be placed ineach region; at low temperatures, a low expansion coefficient isrequired and at higher temperatures (up to a certain limit) higherexpansion coefficients are required. Thus, the thermal expansioncoefficients in the various regions increase in ascending order of thetemperature ranges.

It is to be anticipated that in a graphite moderator structureconstructed in this way any change of dimensions induced by irradiationwill be so small over the working life of the reactor that the graphiteblocks can be arranged to be contiguous over all surfaces at thetemperature of normal operation and that such contiguity will persistduring operation. There is consequently the advantage over previousarrangements employing contiguity on the basis of the temperature beingsufficiently high for the dimensional change to be solely one ofcontraction that play does not develop between the blocks; loss ofrigidity in the structure and inter-channel leakage are thereforeavoided.

Further significant advantages stem from the differences of the neutronflux at various points in a graphite block during reactor operation.Usually there is some increase in the intensity of the flux towards thecentre of a reactor and therefore in some, at least, of the blocks therewill be a flux gradient increasing from the outwardly directed facetowards the inwardly directed face. A nonstable graphite will be subjectdue to this flux variation to a difierential dimensional change whichtends to cause bowing of the block. Another sort of flux variationoccurring in the graphite is the progressive softening of the neutronspectrum with increasing distance from a fuel channel through the block.The differential dimensional change which occurs in non-stable graphitein this case tends to cause cracking of the graphite. The dimensionalstability obtainable by the present invention eliminates these bowingand cracking tendencies.

It will of course be realised that the specification of differentthermal expansion coefficients in different regions demands aconstruction in the form of building units, such as the blocks alreadyquoted. Since it cannot be expected that the shaping of such units tothe contours of the isotherms defining the regions will be convenient,the coefiicient to be employed for any particular unit should be judgedby the region in which most of the unit lies.

Although the foregoing aims at exact determinations for stability, thereare several factors making it difficult to obtain in practice. Inaddition to the fact apparent from the preceding paragraph that steplessvariation of the coefficient cannot be provided, there is likely to beuncertainty in predicting for new graphites that stability will stillprevail when the dose is approaching such a high figure as, say, 100,000mwd./Ate. U (megawatt days per adjacent tonne of uranium) which isbelieved to be feasible for the graphite moderator of thermal nuclearreactors. In particular there is the possibility that the ratio 6 may bea function of dose at high dose figures. There is also the possibilitythat large c axis expansions of the graphite crystals at highirradiation doses may close the porosity normally responsible forreducing the volume thermal expansion coefficient of the bulk materialto below that of a single crystal; this efiect may have the consequenceof varying the constant A However, methods have been devised to enabletesting for A variation Without neutron irradiation so that thesuitability of new graphites in this respect can be easily ascertained.

Having regard to these parctical limitations, it can suffice thatapproximations to the plot in FIG. 1 are applied directly to themoderator graphite of gas-cooled graphitemoderated reactors of the kindin which rod-like fuel elements are disposed in parallel channelspenetrating the moderator. Therefore, as an aid to the dimensionalstabilisation of this graphite, such parts as are subject during normaloperation to a temperature above 325 C. should have a coefficient ofthermal expansion in at least one given direction which is greater than4.5 10- C. measured over the range 196 to +20 C. The nature of thisapproximation tends in the direction to offset possible departures fromdimensional stability at high irradiation doses. A practical limitationis, of course, imposed on the extent to which the quoted figure can beexceeded by the condition that the coefficient cannot be greater thanone third of the thermal expansion coefficient of the crystal in the caxis direction. This isotropy limitation is about 7.3x l0" C. in thepreviously mentioned temperature range. Thus, a coefficientsignificantly above the quoted figure may, for a given temperature ofnormal operation, especially one little above 325 C., lead during theearly stages of the graphite life to expansion under irradiation, thiseffect being preferable to the alternative that with initial stabilitythere comes later a change in behavior whereby contraction occurs.Contraction is apt to create tensile forces to which the graphite isweakly resistant so that cracking can occur.

The possibility of contraction is avoided with greater certainty if, asis preferred, such graphite as is subject during normal operation to atemperature above 300 C. has a coefficient of thermal expansion in atleast one given direction which is greater than 5.0 10- C. in thepreviously mentioned temperature range.

For any graphite in the reactor which has a normal operating temperatureof less than 325 C. the invention further provides on the grounds ofdimensional stability that it has a thermal expansion coefficient whichfor temperatures of normal operation between C. and 240 C. is equal toor greater than and for temperatures of normal operation between 240 and325 C. is equal to or greater than where T is the temperature in degreescentigrade, the coeflicients being measured in the range 196 to +20 C.The preferred rule is that between 130 and 230 C. the thermal expansioncoefiicient is as previously stated, namely equal to or greater than butthat between 230 and 300 C. this coefiicient is equal to or greater thanAs before, the graphite preferably has a random crystal orientation soas to be isotropic to a high degree, at least to the extent of thethermal expansion properties. The graphite should also bewell-graphitized to a high standard of crystallographic perfection.

The lower limits of thermal expansion coefficient quoted above(including those said to be preferred) for the various ranges ofoperating temperatures are derived from FIG. 1, and in the accompanyingdrawing marked FIG. 3 is shown a plot of these lower limits againsttemperature. The line indicated 1 represents the minimum for presentpurposes, the line indicated 2 represents the preferred lower limit andthe line indicated 3 represents a practical upper limit, thislast-mentioned line being defined by [T/400+1.95] 10- C. between 130 and230 C. [T/227.85] 10'/ C. between 230 C and about 300 C., thecoefficient being, as previously, in respect of the range l96 to +20 C.

It is to be noted that FIG. 3 omits the tailing-off at high temperaturesto be sen in FIG. 1. There is irradiation where E is the neutron energy,

(E is the elastic scattering cross section of carbon, and

N (E is the number of displacements produced on average for each neutroncollision, this number being given by d( n) Ed and The values of theparameters for present purposes are:

a=0.284 E =25eV. L =25 x ev.

If the atomic displacement rate for the standard unit upon which FIG. 3is based is denoted I then the corrected temperature T (absolute) can bededuced from:

i l i is) T, T -E I where T, K and B have the same connotation as forEquation 4. Previously it has been pointed that the standard unitadopted herein is represented by a Calder-Hall reactor at 3.12 mw./Ate,which corresponds to an atomic displacement rate of 8.42 10 atoms peratom per second.

Although the foregoing description has specified thermal expansioncoefiicients for the range -196 to C., these coefficients can beconverted as required to the equivalents for other temperature ranges.Equation 1 is used for this conversion and the expression thus derivedfor the coefficient a in respect of the other range is:

where a: is the specified coefficient, a and a,, are the crystalcoeflicients respectively in the c and a axis directions for the samerange as the specified coefficient, and a and er are again the crystalcoetficients but for the other range.

The following table gives certain conversion by way of illustration:

196 to +20 C., per 0.: +100 to +700 C., per C.:

5.0 10 6.7 10 2.0 10- 33x10- +20 to +120 C., per C.:

We claim:

1. In the building of a graphite moderator structure which is exposed toirradiation by neutrons in a nuclear reactor during operation of thereactor, a method of ensuring a high degree of dimensional stability inthe graphite in a given direction comprising the steps of:

(a) determining the temperature distribution throughout the graphitemoderator structure for normal operation of the reactor;

(b) delimiting at least one region of the structure wherein obtains aselected range of normal temperature during normal operation of thereactor;

(c) determining a bulk thermal expansionrcoefiicient for ensuring a highdegree of dimensional stability in a given direction for graphite to beutilized in said delimited region of the structure according to therelationship:

a, (for stability) f wherein:

a (for stability)=the thermal expansion coeflicient of the graphite in adirection x for stability,

a =the thermal expansion coeflicient of a graphite crystal in the c axisdirection,

a,,=the thermal expansion coefiicient of a crystal in the a axisdirection, and

6=the ratio of the rate of irradiation-induced dimensional changes: of agraphite crystal in the a axis direction to the rate ofirradiation-induced dimensional change x in the c axis direction, thevalues of 2: and x being determined by Equations 1 and 2 of thespecification from experimental measurements of bulk dimensional changesand thermal expansion coefficients under experimental irradiation at atemperature representative of said normal temperature range;

(d) fabricating the moderator structure of the reactor by locating, insaid region, graphite having a bulk thermal expansion coefficientcalculated by step (c) to give a high degree of dimensional stability insaid given direction under neutron irradiation at the temperatureobtaining in said region during normal operation of the reactor.

2. A method according to claim 1 wherein the graphite is substantiallyisotropic.

3. A method according to claim 1 wherein the irradiation flux spectrumof said experimental irradiation is different from that of said nuclearreactor and wherein the representative temperature of said region is anequivalent temperature related to an average temperature of said regionprevailing during normal operation of the reactor, the equivalenttempeature being that which, in the neutron flux spectrum of theexperimental irradiations, would produce substantially the same netneutron damage rate in said given direction as will be produced in theneutron flux spectrum of the neuclear reactor in the moderator structurewherein said average temperature prevails.

4. In a nuclear reactor, a graphite moderator structure subject to apredeterminable temperature distribution during normal operation of thereactor and comprising assempled graphite units, said moderatorstructure being delimited into a plurality of regions wherein obtains aselected range of temperature during normal operation of the reactor,and wherein each unit located at least primarily in a given region has abulk thermal expansion coeflicient selected to ensure a high degree ofdimensional stability in a given direction for said graphite in thatdelimited region, said bulk thermal expansion coefiicient being selectedaccording to the following relationship:

graphite a, (for stability) wherein u (for stability)=thethermal'expansion coeflicient of the graphite in a direction x forstability, u =the thermal expansion coefiicient of a graphite crystal inthe c axis direction, a =the thermal expansion coefiicient of a graphitecrystal in the a axis direction, 6=the ratio of the rate ofirradiation-induced dimensional change x,, of a graphite crystal in thea axis direction to the rate of irradiation-induced dimensional change xof a graphite crystal in the c axis direction, the values of x and xbeing determined 1 l by Equations 1 and 2 of the specification fromexperimental measurements of bulk r dimensional changes and thermalexpansion coefficients under experimental irradiation at a temperaturerepresentative of the normal range of said given region.

5. In a nuclear reactor, a moderator structure according to claim 4wherein the graphite is at least substantially isotropic.

6. In a nuclear reactor, a moderator structure according to claim 4wherein each graphite unit lying at least primarily in a given regionhaving a normal operating temperature of at least 325 C. has a bulkthermal ex- 12 pansion coefiicient selected to ensure a high degree ofdimensional stability in a given direction for said graphite in thatdelimited region, said bulk thermal expansion c0- efiicient beingselected from within the range defined by curves 1 and 3 of FIGURE 3.

7. In a nuclear reactor, a moderator structure according to claim 6wherein the graphite is at least substantially isotropic.

No references cited.

REUBEN EPSTEIN, Primary Examiner.

